1. Field of the Invention
The present invention relates to a gas sensor control apparatus which is connected to a gas sensor including a cell for detecting the concentration of a gas to be measured, and which obtains the impedance of the cell to thereby detect the degree of deterioration of the cell.
2. Description of the Related Art
Examples of known gas sensors used for improving fuel consumption of an internal combustion engine of an automobile or the like or for combustion control of the engine include an air-fuel ratio sensor, and an oxygen sensor for detecting the oxygen concentration of exhaust gas. Also, since more rigorous exhaust gas regulations for automobiles demand a reduction in the amount of nitrogen oxides (NOX) contained in exhaust gas, NOX sensors capable of directly measuring the concentration of NOX have been developed.
Each of these gas sensors includes a cell composed of a solid electrolyte body formed of zirconia or the like and having oxygen-ion conductivity, and a pair of electrodes formed on the surface thereof, and detects the concentration of a specific gas on the basis of the output from the cell.
As such a gas sensor, an oxygen sensor is known in which a measurement electrode exposed to a gas to be measured is formed on the outer surface of a solid electrolyte body, a reference electrode exposed to a reference gas is formed on the inner surface of the solid electrolyte body, and the oxygen concentration of the gas to be measured is detected on the basis of a difference between the oxygen concentration of the gas to be measured and that of the reference gas. Since the solid electrolyte body does not exhibit oxygen ion conductivity unless it is heated to its activation temperature or higher, the gas sensor is equipped with a heater. By supplying electric current to the heater to thereby heat the cell, the cell can be stably maintained at a temperature equal to or higher than the activation temperature, and quick activation of the cell becomes possible.
Since the impedance of the cell changes in accordance with the temperature of the cell, this impedance is periodically detected, and the amount of electric current supplied to the heater is controlled such that the impedance coincides with a target impedance, whereby the temperature of the cell is controlled. A signal for impedance detection is input to the cell, and the impedance is detected on the basis of an output (response signal) at that time.
However, there is a problem in that the cell deteriorates with time due to repeated use of the gas sensor, and the impedance thereof increases gradually. Even in the case where the cell is used under the same temperature condition, when the cell deteriorates, the impedance thereof becomes greater than that measured when the cell was new. Therefore, at the time of the temperature control, the deteriorated cell may be determined to still be cool and heated excessively, which may change the output characteristic of the gas sensor, and which may further accelerate deterioration of the gas sensor.
In view of the above-described problem, a technique has been developed of obtaining a high-frequency-side impedance and a low-frequency-side impedance by applying a plurality of AC voltages of different frequencies to a cell, adjusting a target temperature of the cell on the basis of the high-frequency-side impedance, and detecting a change in the characteristic of the cell on the basis of the low-frequency-side impedance (see Patent Document 1). Patent Document 1 describes that, of the two impedances of the cell, the low-frequency-side impedance, which shows the resistance of the electrode interface, increases significantly as a result of deterioration of the cell with time. Thus, the degree of deterioration can be detected and the target temperature of the cell can be adjusted on the basis of a change in impedance when an AC voltage containing a low-frequency component is applied to a new cell and a change in impedance when the AC voltage is applied to a deteriorated cell.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2000-258387 (FIG. 48)